Memory controller that uses a specific timing reference signal in connection with a data brust following a specified idle period

ABSTRACT

Apparatus and methods for operation of a memory controller, memory device and system are described. During operation, the memory controller transmits a read command which specifies that a memory device output data accessed from a memory core. This read command contains information which specifies whether the memory device is to commence outputting of a timing reference signal prior to commencing outputting of the data. The memory controller receives the timing reference signal if the information specified that the memory device output the timing reference signal. The memory controller subsequently samples the data output from the memory device based on information provided by the timing reference signal output from the memory device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior U.S. patent application Ser.No. 13/105,798, filed on May 11, 2011 for “Protocol including acommand-specified timing signal,” said prior application hereby beingincorporated by reference. U.S. patent application Ser. No. 13/105,798,in turn, claims the benefit of U.S. Provisional Patent Application No.61/366,806, filed on Jul. 22, 2010, also for “Protocol including acommand-specified timing signal.”

TECHNICAL FIELD

The present embodiments generally relate to techniques for communicatingdata between integrated circuit devices. More specifically, the presentembodiments relate to a method, apparatus and protocol for high speedsignaling between a memory device and a memory controller.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A presents a block diagram illustrating a system which performstiming updates between a memory controller and at least one memorydevice.

FIG. 1B presents a block diagram illustrating an exemplary system whichperforms fast timing updates between a memory controller and a set ofmemory ranks.

FIG. 2 presents a simplified timing diagram illustrating preamble-basedfast timing updates between a memory controller and a rank.

FIG. 3 presents a timing diagram illustrating preamble-based fast timingupdates during read memory transactions on multiple ranks.

FIG. 4A presents a timing diagram illustrating preamble-based fasttiming updates during both write and read memory transactions onmultiple ranks.

FIG. 4B presents a timing diagram illustrating preamble-based fasttiming updates during read memory transactions using a dedicatedsignaling resource for TRS/EDC modes.

FIG. 5 presents a block diagram illustrating an embodiment of a memorysystem, which includes at least one memory controller and one or morememory devices.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular example application and its requirements. Variousmodifications to the disclosed embodiments will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to other embodiments and applications without departing fromthe spirit and scope of the present invention. Thus, the presentinvention is not limited to the embodiments shown, but is to be accordedthe widest scope consistent with the claims.

For computer systems, such as servers which contain numerous ranks ofmemory, idle power may be an issue. At any given time at most one rankis actively accessed, while all other ranks are idle but still consumingidle power. In some servers, this idle power can exceed active power ifthere are a large number of idle ranks. In such systems, power can besaved by moving timing control components, such as phase-locked loop(PLL) or delay-locked loop (DLL), from the ranks to the memorycontroller (which typically controls multiple ranks). In such designs,timing information can be provided to the memory controller fromindividual ranks to make timing updates to compensate for the noise anddrift of the ranks. Such timing information can be conveyed via datasignaling resources, for example, as a timing preamble that precedes adata burst received by the controller via the data signaling resources.Other signaling resources that are common to a given rank of memorydevices and the controller may also be used to transfer the timinginformation between the rank and the controller.

Timing information received by the memory controller for a given rank isavailable when the rank is actively accessed (e.g., through a read orwrite command). By utilizing certain common timing reference/dataresources for multiple ranks, when accessing a given rank, no timinginformation may be received from other ranks in the memory system.During this time, supply voltage and temperature may drift, which cancause the delay of the circuitry in the individual ranks to change.Moreover, high-frequency power supply noise can also shift the timing ofthe ranks. Hence, if a rank has not been accessed for some period oftime, it is necessary to first perform a timing update for the rankbefore the rank can be accessed.

The following description presents various exemplary methods, protocolsand apparatus for performing timing updates for memory transactionsbetween a memory controller and a memory device (e.g., in a multi-ranksystem). In some embodiments, at the beginning of accesses to memorydevices in a memory rank (or a “rank”), for example, after the rank hasbeen idle for a time period, the memory controller transmits, to amemory device, a read command containing information which specifieswhether the memory device is to output a timing reference signal (“TRS”)prior to outputting of data in response to the read command. In anembodiment, the information is a digital bit or a special flag and isreferred to as a preamble request flag. When the memory device, uponcondition that the information specifies a TRS, outputs the TRS to thecontroller, the controller will receive the TRS before receiving a firstbit of the subsequent data. The controller, upon receipt of the TRS, canuse the TRS for the sampling of the data. For example, the TRS is usedto sample the data or, as is described in more detail below, thecontroller can derive phase information from the TRS to optimize asampling point for the data at the input sampler of the controllerdevice. For example, the controller can update the phase of an internalsampling clock based on phase information derived from the TRS, wherethe internal sampling clock is an internal timing reference provided toan input sampler, in the controller, used to sample incoming read dataoutput by the memory device.

In an embodiment, the TRS is a calibration preamble and in response tothe information specifying a calibration preamble, the memory devicetransmits the calibration preamble from the memory device to the memorycontroller. The memory controller subsequently uses the calibrationpreamble to adjust receive timing for sampling, at the memorycontroller, the data output from the memory device. The receive timingat the memory controller may be adjusted to compensate for low-frequencytiming drifts and timing jitter.

In some embodiments, the TRS has a burst length which is determined inpart by the duration of an idle period for the rank. The burst length ofthe timing reference signal may be set to be the same as the burstlength of the data (output in response to the read command). The TRS, inan embodiment, is conveyed from the memory device, to the memorycontroller via a signaling resource (i.e., signal traces, pins etc.,)that is also used to transfer error detection and correction information(EDC). Over the signaling resource, the timing reference signal may beconveyed in a time shifted manner relative to the data burst beingtransferred over a separate signaling resource.

In some embodiments, the calibration preamble is only requested when thefirst transaction to a newly accessed rank is a read access. In thediscussion below, the terms “timing updates” and “phase updates” areused interchangeably, and the terms “timing reference signal (TRS)”,“calibration preamble” and “preamble” may also be considered as beingused interchangeably. In addition, “ranks” are used to refer to 1 ormore memory devices which are accessed and (collectively in the case ofmore than 1 memory device) transfers data for a given data accessoperation.

FIG. 1A presents a block diagram illustrating a system 100 whichperforms timing updates between a memory controller 102 and a set ofmemory ranks (such as DRAM ranks) 104. Memory controller 102 includes aPLL circuit 106 and an edge tracking circuit 108. Memory ranks 104include two or more ranks, such as a rank 110. In an embodiment, eachrank 110 includes one or more memory devices 111 and a clockdistribution circuit 112, and each memory device 111 is a single chiphaving a memory core 113.

During a memory access to an active rank 110, memory controller 102transmits command/address (C/A) 114 and command (CA) clock 116 to rank110 over a chip-to-chip interconnect 118. Memory controller 102 can alsotransmit data 120 and data clock 122 to rank 110 over chip-to-chipinterconnect 118. In some embodiments, data clock 122 is a full-bit-ratedifferential data clock (DCLK) for the data interface on rank 110, whichprovides a clock edge per bit, whereas CA clock 116 is a differentialclock operating at one-quarter the rate of data clock 122, which is usedfor timing on the CA interface and for memory core 111. Note that thememory device CA input samplers (not shown) on rank 110 are directlyclocked by CA clock 116, while write data 120 is directly clocked onrank 110 by data clock 122.

Memory rank 110 also receives data 120 and data clock 122, anddistributes data clock 122 through clock distribution circuit 112 toboth an input/output (I/O) data slicer 124 and an error detection andcorrection (EDC) circuit 126. I/O data slicer 124 includes a transmittercircuit to transmit data 128 and EDC circuit 126 includes a transmittercircuit to transmit an EDC signal 130 back to memory controller 102.Note that both data 128 and EDC signal 130 contain phase informationregarding the memory transactions taking place on rank 110. In someembodiments, EDC signal 130 contains CRC codes, which are interspersedwith a toggling idle pattern to achieve a minimum or predeterminednumber of EDC signal edge transitions (edge density).

Edge tracking circuit 108 on memory controller 102 receives both data128 and EDC signal 130, and subsequently uses clock-data-recovery(CDR)-like circuits to extract phase information from the received datafor rank 110. This phase information is then sent to PLL circuit 106,which uses the phase information to perform phase updates for data clock122 to compensate for timing drift and timing jitter for rank 110. PLLcircuit 106 also updates CA clock 116 based on the same phaseinformation from edge tracking circuit 108.

In an embodiment, to facilitate the memory controller switching betweenservicing read operations directed to different ranks, phase informationfor each individual rank can be stored at the memory controller. Forexample, in the embodiment illustrated in FIG. 1A, memory controller 102includes a rank-tracking circuit 132 coupled to edge-tracking circuit108, which stores phase information for each of the memory ranks 104.Hence, the phase information for a new rank to be accessed is firstloaded into a phase mixer after a rank switch occurs. At the same time,the most recently updated phase information for a previously accessedrank is stored into rank-tracking circuit 132.

When a new rank is accessed after an idle period for the rank, data andan EDC signal eventually become available from the new rank. It ispossible to use the EDC signal to perform timing updates for the newrank using the above-described timing update loop in FIG. 1A. However,this timing update loop involves feedback and a long path delay whichmay not be sufficiently fast to track high-frequency timing jitter, suchas power supply induced jitter (PSIJ). Moreover, the EDC signal onlycaptures timing errors for preceding transactions, and therefore trailsbehind the data used to produce the EDC signal. Hence, during readtransactions, the memory controller only receives the EDC signal afterread data has already been received without associated clock phaseupdates.

Some of the presently described techniques enable rapid phase updatesfor a rank which is being accessed for the first time after an idleperiod. This is accomplished by initially transmitting a timingreference signal (TRS) from the rank to the memory controller before thefirst read data packet and the corresponding EDC packet are transmitted.In some embodiments, the timing reference signal may be added to thehead of the first available read data burst being transmitted from anewly accessed rank to the memory controller. Upon receiving the TRS,the memory controller may perform fast phase updates within the durationof the TRS to correct a substantial portion of the timing drift andjitter for the rank. In some embodiments, this TRS comprises a togglingpattern which provides a predetermined number of timing edges.

FIG. 1B presents an example embodiment illustrating system 129 whichperforms fast timing updates between a memory controller 102 and a setof memory ranks 104. As illustrated in FIG. 1B, a fast edge trackingcircuit 136 is included in memory controller 102's clock architecture toprovide fast phase updates within a received calibration preamble. Forexample, assume fast edge tracking circuit 136 receives data 128 fromthe new rank being accessed. If data 128 contains a calibrationpreamble, fast edge tracking circuit 136 uses the toggling pattern inthe calibration preamble to make fast phase updates to a data clock usedto capture the actual read data in data 128 following the calibrationpreamble. In this embodiment, after the initial updates based on thecalibration preamble have been made, edge tracking circuit 108 performssubsequent phase updates during data transactions following thecalibration preamble. In some embodiments, fast edge tracking circuit136 operates at a significantly faster clock rate than edge trackingcircuit 108, which allows the phase update to complete within a preambleduration, before the actual read data arrives at the memory controller.Furthermore, to ensure their independent operation without interferingwith each other, fast edge tracking circuit 136 and edge trackingcircuit 108 can be gated by different control signals, namely fast lockenable 138 and EDC lock enable 140 in FIG. 1B.

The additional phase adjustment capability provided by fast edgetracking circuit 136 in the clock architecture of memory controller 102facilitates additional noise tracking and can relax the designrequirements for the clock distribution circuit 112 on memory ranks 104.Moreover, fast edge tracking circuit 136 can have a shorter adjustmentrange for accommodating the timing drift expected from the idle periodbetween accesses to a given rank. This amount of drift may also bebounded by specifying a maximum allowable idle period between accessesto a given rank.

FIG. 2 presents a simplified timing diagram illustrating preamble-basedfast timing updates between a memory controller and a rank in accordancewith an embodiment. As illustrated in FIG. 2, a data signal DQ[n] 202comprises the beginning of a data pattern received at a memorycontroller from a new rank [n] being accessed after an idle period.DQ[n] 202 begins with a calibration preamble 204 comprising a togglingpattern which contains a predetermined number of edges. Preamble 204 isimmediately followed by a data burst 206 which has a predetermined burstlength for example, 4, 8, 16, 32 bits or 64 bits. An EDC signal 208associated with data burst 206 is also received by the memory controllerfrom the same rank. Note that EDC signal 208 trails both preamble 204and data burst 206.

EDC signal 208 contains EDC codes 210 interspersed with toggling idlepatterns 212 to achieve a minimum edge density (i.e., a predeterminednumber of signal transitions during a given time interval) in EDC signal208. EDC codes 210 can include any type of EDC code, such as a CRC code.Note that EDC codes 210 can be of different lengths and can occur indifferent orders with the idle patterns or without the idle patterns atall. While the edge density in EDC codes 210 can be guaranteed by usingthe idle patterns, other techniques (e.g., various coding styles on topof the EDC codes) can be used to accomplish the same. In someembodiments, no special technique is used in EDC codes 210, and thesystem relies on probability to provide enough edges.

Upon receiving DQ[n] 202, the memory controller uses preamble 204 toperform a fast phase update without using a time-consuming feedbackmechanism. A fast lock enable signal 214, which has an enable window 216substantially time-aligned with preamble 204, is used to gate the fastupdates at a fast clock rate. After completing the fast phase updates,normal phase updates can be performed based on EDC signal 208. Morespecifically, an EDC lock enable 218 which has an enable window 220substantially time-aligned with EDC code 210 is used to gate the normalphase updates at a normal slow clock rate which may involve a feedbackloop. In some embodiments, however, the enable window 216 of fast lockenable 214 may be extended (as shown by the dotted line) to thebeginning of enable window 220 in EDC lock enable 218. In thisembodiment, the initial phase updates (before the EDC-based update)include not only the fast-lock adjustments from preamble 204, but alsosome phase adjustments based on the first portion of data burst 206.

FIG. 3 presents a timing diagram illustrating preamble-based fast timingupdates during read memory transactions on multiple ranks according toan embodiment. As illustrated in FIG. 3, a memory controller issuesmemory commands (CA), which include two consecutive read commands RD[A0]and RD[A1] directed to a rank 0. Note that RD[A0] is the first memoryaccess request issued to rank 0 after an idle period for rank 0transpires. In response, a memory device in rank 0 prepends acalibration preamble 302 to read data Q[A0].

After the controller issues read command RD[A1], a rank switch occursfrom rank 0 to idle rank 1. More specifically, immediately after issuinga precharge command PRE to close off the access to rank 0, twoconsecutive read commands RD[A2] and RD[A3] are issued to rank 1,wherein RD[A2] is the first memory access request on rank 1 after theidle period on rank 1. In response, rank 1 prepends a calibrationpreamble to read data Q[A2].

Some embodiments use a “read with preamble” command to support timingupdates in the controller clock architecture when the memory controlleraccesses a rank of memory that has not been accessed for some period oftime. More specifically, the memory controller is configured to identifya condition when a rank is to be accessed for the first time after anidle period, and the first memory access to the rank is a read. Forexample, this condition occurs immediately before RD[A0] and RD[A2] inFIG. 3. When this condition is identified, the memory controller issuesthe read command containing a preamble request flag to the rank. Uponreceiving the read command at the rank, the preamble request flag causesthe rank to send a calibration preamble before transmitting therequested read data to the memory controller. The memory controller thenuses the calibration preamble to update the read clock phase at thememory controller.

In one embodiment, the preamble request flag is a 1-bit field in theread command. For example, a ‘1’ provided in the 1-bit field providesfor a “read with preamble” command, and a ‘0’ provided in the 1-bitfield provides for a “read without preamble” command. In otherembodiments, the preamble request flag is encoded in a multi-bit fieldin the read command and could be combined with other command variants ofthe read command, for example, a read with/without auto precharge bitthat specifies whether sense amplifiers of the memory device should beautomatically precharged following the read operation. Another commandvariant of the “read with preamble” command is a “preamble without data”command that specifies that the memory device output a timing referencesignal to the controller device with no succeeding data burst beingaccessed during the transaction with memory. This command optionprovides that after a predetermined idle period to an idle rank, thememory controller flexibly has the option to schedule a receive timingupdate to that idle rank by specifying that a memory device of the ranktransmit a timing reference signal with no accompanying data burst.

Generally, the TRS is predetermined to provide enough edges to performthe required timing updates before the read data is captured. The lengthof the TRS may be specified by a field in the read with preamble commandor “statically” programmed to accommodate a given memory controllerarchitecture and can be stored in registers on target memory devices ofa rank and/or the memory controller. For example, some preamble burstlengths can be programmed to be 4, 8, 16, and 32 clock cycles. In oneembodiment, two or more length values are programmed in registers on thetargeted memory device and a field in the command selects from the twoor more length values.

In some embodiments, a preamble request command includes a command field(one or more bits long) specifying the length of the TRS to allow thememory controller to specify the length of the preamble as needed basedon various factors, such as the amount of elapsed time since the lastrank access. In some embodiments, the memory controller can determinethe length of the preamble based at least in part on the idle time of agiven rank since the last access (i.e., an idle time-based length). Itselects a shorter length for a shorter idle time and a longer length fora longer idle time. For example, the controller can use a one-bitcommand field to determine whether to send a long (e.g., 32 cycles)preamble or a short (e.g., 16 cycles) preamble. In another embodiment,the controller can simply use a bit of the command field to specifywhether to perform a fine adjustment with a long preamble, or a fastadjustment with a short preamble based on run-time factors associatedwith the operation of the memory system.

The calibration preamble may be only needed for the first readtransaction after an idle period; once the timing updates have beencompleted, any further transactions to the active rank will use EDCedges to maintain phase updates. Note that if the first memorytransaction to a previously idle rank is a write transaction, the readtiming can be automatically updated through the edges returned on theEDC line. We describe this situation in more detail below.

FIG. 4A presents a timing diagram illustrating preamble-based fasttiming updates during both write and read memory transactions onmultiple ranks. As illustrated in FIG. 4A, a memory controller issues amemory command (CA), which includes four consecutive memory accessrequests: A0-A1, which are directed to rank 0, and A2-A3, which aredirected to rank 1. The first memory access command RD[A0] is issued torank 0, wherein RD[A0] is the first memory access request on rank 0after an idle period for rank 0. In response, rank 0 prepends acalibration preamble 402 to read data Q[A0] returned for RD[A0].

Next, a write command WR[A1] is issued to rank 0 which causes a busturnaround 404. Moreover, WR[A1] is the first write to rank 0 after thememory controller switches rank to rank 0. In this embodiment, rank 0does not send another calibration preamble prior to receiving write dataD[A1] from the memory controller. As illustrated in FIG. 4A, while D[A1]is being transmitted, EDC data EDC[A0] associated with read data Q[A0]is being received by the memory controller. Hence, the memory controllercan use EDC[A0] to perform phase updates both during bus turnaround 404and at the beginning of transmitting write data D[A1] to rank 0. Also,note that since there is not a rank switch between the read to A0 andwrite to A1, the EDC signal may alternatively continue to toggle orotherwise provide an “idle pattern” during the bubble period between A0and A1 as is illustrated by the arrow in FIG. 4A. This idle patterncould be pre-defined or register programmable. It may also be commanddriven. For example, an extra command on the CA bus may actually be usedto signal the DRAM that it will continue to be the rank that “owns” thedata bus. (The DRAM could determine this itself based on its internalstate machine, given that the write command has already been received bythe DRAM prior to the “bubble” period.)

After issuing write command WR[A1], the memory controller switches ranksfrom rank 0 to idle rank 1. More specifically, immediately after issuinga precharge command PRE to close off the access to rank 0, a writecommand WR[A2] is issued to rank 1, wherein WR[A2] is the first memoryaccess request on rank 1 after the idle period on rank 1. In theembodiment of FIG. 4A, rank 1 does not send a calibration preamblebefore receiving write data D[A2] from the memory controller.

In an alternative embodiment to FIG. 4A, before issuing the writecommand WR[A2] to rank 1 after the rank switch, the memory controllercan send a “preamble only” command to rank 1. This preamble only commandcauses rank 1 to send only a calibration preamble with a predeterminednumber of edges to the memory controller. The memory controllersubsequently uses the calibration preamble to determine the amount oftiming drift (due to voltage, temperature, etc.) that has occurred onrank 1. The memory controller can then compensate for the timing driftprior to transmitting the write data D[A2] to rank 1.

In a variation to the preamble only command technique, before issuingthe write command WR[A2] to rank 1, the memory controller can issue an“address-less” or “data-less” read command to trigger a calibrationpreamble to be generated by rank 1 (because this is the first readcommand after the memory controller starts accessing rank 1). As aresult, the calibration preamble is sent to the memory controllerwithout causing any read data to be sent.

Note that, similar to the read with preamble command, the data-lesspreambles can have their length specified by a field in the “data-less”read command or “statically” programmed in a register on the targetedmemory device. In one embodiment, two or more length values areprogrammed in registers on the targeted memory device and a field in thecommand selects from the two or more length values.

Referring back to FIG. 4A, note that after sending write command WR[A2]to rank 1, a read command RD[A3] is subsequently issued to rank 1, whichcauses another bus turnaround 406. In this case, RD[A3] is the firstread command directed to rank 1 after the memory controller switched torank 1. In this situation, rank 1 does not send a calibration preamblebefore sending read data Q[A3] to the memory controller. This is becauseif the first memory transaction is a write (e.g., WR[A2]), the memorycontroller will already have received phase information from the EDCsignal (e.g., EDC[A2]) during the write transaction, thereby allowing anupdate to the read clock phase, before a subsequent read transaction(e.g., RD[A3]) is initiated. As illustrated in FIG. 4A, while WR[A2] isin progress, EDC data EDC[A2] (associated with write data D[A2]) isreceived by the memory controller. Hence, the memory controller can useEDC[A2] to perform phase updates both during bus turnaround 406, and atthe beginning of read data Q[A3] from rank 1.

Note that, while the embodiments illustrated in FIGS. 3 and 4 onlyillustrate two memory transactions on a given rank, subsequent memorytransactions to the same rank do not require a calibration preamble. Forexample, a series of accesses to the same rank can commence with a readwith preamble command (i.e. the first memory access after an idle periodon the rank) followed by a write command, which is then followed by aread without preamble command.

In one embodiment, the memory controller can detect ranks that have notbeen accessed for a long period time. To prevent a large timing drift,which may be difficult to correct with a reasonable length preamble, thecontroller can periodically “talk” to a rank if the rank has been idlelonger than a predetermined maximum time by sending a read with preamblecommand or a data-less read command to that rank. By sending suchcommands periodically, the memory controller avoids the need to performlater calibration operations that would necessitate longer calibrationpreambles, or a more exhaustive calibration process, such as oneperformed upon initialization of the system.

The above-described techniques and apparatus can be used in differentsystems having chips that that communicate with one another viachip-to-chip interfaces. Examples of such systems include front-sidebuses, and systems employing different types of memory devices andmemory controllers that control the operation of these memory devices.Examples of these systems include, but are not limited to, mobilesystems, desktop computers, servers, and/or graphics applications. Thememory devices can include dynamic random access memory (DRAM).Moreover, the DRAM may be, e.g., graphics double data rate (GDDR, GDDR2,GDDR3, GDDR4, GDDR5, and future generations) and double data rate (DDR2,DDR3 and future memory types).

The techniques and apparatus described may be applicable to other typesof memory, for example, flash and other types of non-volatile memory andstatic random access memory (SRAM). Moreover, throughout thisdescription, a clock signal is described; it should be understood that aclock signal in the context of the instant description may be embodiedas a strobe signal or, generally, as a signal that conveys timinginformation, and is not limited to a signal that is strictly periodic.For example, a timing reference may be a clock signal, which is periodic(however, it may be gated), or may be a strobe signal (that is aperiodicin the sense that it indicates when to sample data)

In several embodiments of the instant description, the error detectionand correction (EDC) and timing reference signal (TRS) are illustrated,with the TRS signal sharing a signaling resource (pin, interconnectetc.) with data and with the EDC signal conveyed over a dedicated EDCsignal line between the memory controller and a memory device. In otherembodiments, the EDC signal and the TRS signal can each be conveyed,over respective dedicated signaling resources, between the memorycontroller and a memory device. For example, during a read operation,the TRS signal is transmitted by a memory device over a first signalline and is to be received by the memory controller before the read datais received (over separate signal lines) so that the memory controllercan adjust its timing alignment to properly receive the read data. Forthe EDC operation corresponding to the memory read operation, the EDCsignal is provided from the memory device over a second signal line andis skewed to be later in time than the resulting read data output sincethe EDC codes must be calculated before being transmitted from thememory device to the memory controller.

To prevent the EDC and/or TRS signals provided from different ranks frominterfering with each other, additional spacing may be introducedbetween reads operations from different ranks. In an embodiment, toimprove system bandwidth that may be otherwise consumed as a result ofthe additional spacing, the memory device limits the duration of the TRSand EDC signals for each transaction to be equal to the data burstlength for the transaction (either read or write.) The TRS and EDCsignals and read data have the same burst length and are skewed in timefor their particular operation. For example, the TRS signal associatedwith a read operation is skewed to be earlier than the correspondingdata in order to provide sufficient time for the memory controller toacquire timing information from the TRS signal before the data burst ofthe read operation begins. In this embodiment, the memory device beginssending the TRS signal between the time when it receives the readcommand (“read with TRS”) and the time at which the memory devicetransmits the first bit of data of the corresponding read data. Inaddition, the memory device skews the timing of the EDC transmissioncorresponding to the read data to commence later than the time at whichthe memory device transmits the first bit of data.

In an embodiment, both the EDC and TRS are conveyed (either in TRS modeor in EDC mode) over the same signal line between the memory device andthe memory controller. FIG. 4B presents a timing diagram illustratingpreamble-based fast timing updates during read memory transactions usinga dedicated signaling resource for TRS/EDC modes. Specifically, FIG. 4Billustrates memory read transactions with a TRS/EDC signaling resourceconfigured in TRS mode, where a TRS signal is output by the memorydevice, prior to a corresponding first bit of data being output by thememory device. A transmitter circuit on the memory device provides theTRS/EDC to the memory controller via the TRS/EDC signaling resource inaccordance with the mode programmed by the memory controller, orspecified by the memory controller on a per read transaction basis.

In an embodiment, a register on the memory device stores a value whichspecifies either TRS mode or EDC mode. In this example, the timing ofwhen each of the TRS and EDC signals are transmitted is shifteddepending on which signal is being transmitted. For example, in TRSmode, the TRS signal burst would be shifted to commence before thecorresponding read data burst commences, for example, as is illustratedin FIG. 4B, and in EDC mode, the EDC signal would be shifted to commenceafter corresponding read data burst commences. In this embodiment and inembodiments described above, the memory device may set the burst lengthof the TRS and EDC signals for each transaction to be equal to the databurst length for the memory transaction (either read or write). Theburst length for the memory transaction is stored as a representativevalue in a programmable register on the memory device.

In various embodiments, registers disposed on the memory device storevalues that represent time intervals between TRS/EDC and datatransmission. In more detail a first register, disposed on the memorydevice, may be programmed by the controller via a set register commandaccompanied by a first value that the memory device receives and storesin the first register. The first value represents a first time intervalthat transpires between a referenced point of the timing referencesignal (e.g., a first toggle of the TRS burst, or last toggle of the TRSburst) and the beginning, or first bit of the data burst. The first timeinterval may be, for example, be expressed as a number of clock cyclesor a number of nano seconds that transpire.

For a data to EDC time interval, a second register, disposed on thememory device, may be programmed by the controller via the set registercommand accompanied by a second value that the memory device receivesand stores in the second register. The second value represents a firsttime interval that transpires between a referenced point of the databurst (e.g., launching of the first bit of data) and, for example, thebeginning, or first bit of the EDC burst. The second time interval may,for example, be expressed as a number of clock cycles or a number ofnanoseconds that transpire.

In another embodiment, the functionality of the first and secondregisters is served by a single register that specifies a time intervalthat is applied, depending on which mode the memory device is programmedto be in. For example, a single register, in this embodiment, contains avalue that represents a timing interval that separates TRS and dataoutput for TRS mode, and specifies a timing interval that separates dataand EDC output for EDC mode.

In another embodiment, the controller can specify individual burstlengths for the TRS and EDC signals via corresponding programmableregisters on the memory device. These registers may be programmed via aregister program command in which values that represent burst lengthsfor the TRS and EDC signals are conveyed from the controller and storedin their respective registers. Alternatively, the EDC and TRS burstlengths may be specified by information that is provided with the memoryaccess commands (e.g., Read and Write commends) for a correspondingmemory access. In another embodiment, multiple sets of programmableregisters are implemented on a memory device. Each set of programmableregisters stores values that are representative of respective differentselectable EDC burst lengths and TRS burst lengths. Information providedwith a memory access command specifies which register is selected toprovide the burst length for the EDC and TRS signals.

Additional embodiments of memory systems that may use one or more of theabove-described apparatus and techniques are described below withreference to FIG. 5. FIG. 5 presents a block diagram illustrating anembodiment of a memory system 500, which includes at least one memorycontroller 510 and one or more memory devices 512 arranged in each rank.Memory controller 510 and each memory device 512 are individualintegrated circuit devices, which may be included in a singlechip-package. For example, memory controller 510 may be stacked with oneor more memory devices 512 in a through-silicon via (TSV) stack.Alternatively, multiple memory devices 512 may be integrated in a TSVstack, such that one of them may act like a “memory controller” whichrequests TRS from other memory devices 512 in the stack.

In some embodiments, memory controller 510 is a local memory controller(such as a DRAM memory controller) and/or is a system memory controller(which may be implemented in a microprocessor, an application-specificintegrated circuit (ASIC), a system-on-a-chip (SoC) or afield-programmable gate array (FPGA)).

Memory controller 510 may include an I/O interface 518-1 and controllogic 520-1. In some embodiments, one or more of memory devices 512include control logic 520 and at least one of interfaces 518. However,in some embodiments some of the memory devices 512 may not have controllogic 520. Moreover, memory controller 510 and/or one or more of memorydevices 512 may include more than one of the interfaces 518, and theseinterfaces may share one or more control logic 520 circuits. In someembodiments one or more of the memory devices 512 is configured as amemory rank 516.

As discussed in conjunction with FIGS. 1A, 1B, 2, 3 and 4, control logic520-1 on memory controller 510 may be used to issue “read with preamble”commands to ranks within memory devices 512 to request timing referencesignals for fast timing updates when these ranks are being accessed forthe first time after having been idle. Memory controller 510 can alsodetermine a suitable length for each TRS being requested. Note that thetechniques described in conjunction with FIGS. 1A, 1B, 2, 3 and 4 mayalso be applied in systems with only one rank of memory, when thissingle rank has not been accessed for some period of time, thusrequiring the same timing update as described for a multi-rank system.

Memory controller 510 and memory devices 512 are coupled by one or morelinks 514, such as multiple wires, in a channel 522. While memory system500 is illustrated as having three links 514, other embodiments may havefewer or more links 514. Moreover, these links may provide: wired,wireless and/or optical communication. Furthermore, links 514 may beused for bidirectional and/or unidirectional communication between thememory controller 510 and one or more of the memory devices 512. Forexample, bidirectional communication between the memory controller 510and a given memory device may be simultaneous (full-duplexcommunication). Alternatively, the memory controller 510 may transmitinformation (such as a data packet which includes a command) to thegiven memory device, and the given memory device may subsequentlyprovide the requested data to the memory controller 510, e.g., acommunication direction on one or more of the links 514 may alternate(half-duplex communication). Also, one or more of the links 514 andcorresponding transmit circuits and/or receive circuits may bedynamically configured, for example, by one of the control logic 520circuits, for bidirectional and/or unidirectional communication.

Signals corresponding to data and/or commands (such as request-for-datacommands) may be communicated on one or more of the links 514 usingeither or both edges in one or more timing signals. These timing signalsmay be generated based on one or more clock signals, which may begenerated on-chip (for example, using a phase-locked loop and one ormore reference signals provided by a frequency reference) and/oroff-chip. Note that the CA bus on links 514 may be used to conveysignals having predetermined functional definitions (e.g. Bank address,Write enable, Chip select, etc.) or it may be packetized on a smallerset of pins.

In some embodiments, commands are communicated from the memorycontroller 510 to one or more of the memory devices 512 using a separatecommand link, i.e., using a subset of the links 514 which communicatecommands. However, in some embodiments commands are communicated usingthe same portion of the channel 522 (i.e., the same links 514) as data.Moreover, communication of commands: may have a lower data rate than thedata rates associated with communication of data between the memorycontroller 510 and one or more of the memory devices 512; may usedifferent carrier frequencies than are used to communicate data; and/ormay use a different modulation technique than is used to communicatedata.

Devices and circuits described herein may be implemented usingcomputer-aided design tools available in the art, and embodied bycomputer-readable files containing software descriptions of suchcircuits. These software descriptions may be: behavioral, registertransfer, logic component, transistor and layout geometry-leveldescriptions. Moreover, the software descriptions may be stored onstorage media or communicated by carrier waves.

Data formats in which such descriptions may be implemented include, butare not limited to formats supporting behavioral languages like C,formats supporting register transfer level (RTL) languages like Verilogand VHDL, formats supporting geometry description languages (such asGDSII, GDSIII, GDSIV, CIF, and MEBES), and other suitable formats andlanguages. Moreover, data transfers of such files on machine-readablemedia may be done electronically over the diverse media on the Internetor, for example, via email. Note that physical files may be implementedon machine-readable media such as: 4 mm magnetic tape, 8 mm magnetictape, 3½ inch floppy media, CDs, DVDs, and so on.

The foregoing descriptions of embodiments of the present invention havebeen presented only for purposes of illustration and description. Theyare not intended to be exhaustive or to limit the present invention tothe forms disclosed. Accordingly, many modifications and variations willbe apparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention. The scope ofthe present invention is defined by the appended claims.

1. A method of operation in a memory controller, the method comprising:transmitting a read command which specifies that a memory device outputdata accessed from a memory core via a data bus, the data to be outputby the memory device as a burst of data words; provided that the burstof data words follows a period of specified duration during which thememory device was not exchanging data with the memory controller,transmitting information to the memory device which specifies whetherthe memory device is to commence outputting of a specified timingreference signal prior to commencing outputting of the burst of datawords; and receiving the timing reference signal if the informationspecified that the memory device output the timing reference signal. 2.The method of claim 1, wherein transmitting the information to thememory device comprises programming a register of the memory device tocause the outputting of the specified timing reference signal prior tocommencing outputting of the burst of data words.
 3. The method of claim1, wherein the timing reference signal comprises a toggling patternhaving at least four bits.
 4. The method of claim 3, wherein the methodfurther comprises receiving the toggling pattern at the memorycontroller and using the toggling pattern to perform at least one phaseupdate of a timing signal used to exchange data with the memory devicevia the data bus.
 5. The method of claim 3, wherein the memory device isa first memory device and corresponds to a first rank of memory, whereinthe memory controller is also to exchange data with a second memorydevice which corresponds to a second rank of memory via the data bus,and wherein the transmitting of the information is performed in theevent that the read command is to result in a switch of ranks exchangingdata with the memory controller via the data bus.
 6. The method of claim3, wherein the toggling pattern is timed in dependence on a clocksignal, and wherein the method further comprises transmitting the clocksignal from the memory controller to the memory device.
 7. The method ofclaim 1, wherein the timing reference signal is to be added to the headof the first available data associated with the burst of data words. 8.The method of claim 7, wherein the transmitting of the information tothe memory device comprises transmitting the information as part of theread command.
 9. The method of claim 1, wherein the method comprisessampling the burst of data words output from the memory device at thememory controller based on information provided by the specified timingreference signal output from the memory device.
 10. A memory controllerintegrated circuit, comprising: circuitry to transmit a read command toa memory device, the read command to cause the memory device to outputdata accessed from a memory core via a data bus, the data to be outputby the memory device as a burst of data words; circuitry to, providedthat the burst of data follows a period of specified duration duringwhich the memory device was not exchanging data with the memorycontroller integrated circuit via the data bus, transmit information tothe memory device which specifies whether the memory device is tocommence outputting of a specified timing reference signal prior tocommencement of outputting of the burst of data; and receiver circuitryto receive the timing reference signal if the information specified thatthe memory device output the timing reference signal.
 11. The memorycontroller integrated circuit of claim 10, wherein circuitry to transmitthe information to the memory device is to issue a register programmingcommand to the memory device to program a register of the memory deviceto cause the output the specified timing reference signal prior tocommencing output of the burst of data words.
 12. The memory controllerintegrated circuit of claim 10, wherein the timing reference signalcomprises a toggling pattern having at least four bits.
 13. The memorycontroller integrated circuit of claim 12, wherein the memory controllerintegrated circuit further comprises phase update circuitry to update atleast one phase value in response to receipt of the toggling pattern bythe memory controller integrated circuit, said memory controllerintegrated circuit to use the at least one phase update of a timingsignal used to exchange data with the memory device via the data bus.14. The memory controller integrated circuit of claim 12, wherein thememory device is a first memory device and corresponds to a first rankof memory, wherein the memory controller integrated circuit is furtherto exchange data with a second memory device which corresponds to asecond rank of memory via the data bus, and wherein the circuitry totransmit of the information to the memory device is to do so in theevent of a rank switch.
 15. The memory controller integrated circuit ofclaim 12, wherein the toggling pattern is to be timed dependent on aclock signal, and wherein the memory controller integrated circuitfurther comprises clock forwarding circuitry to provide the clock signalto the memory device.
 16. The memory controller integrated circuit ofclaim 10, wherein the timing reference signal is to be added to the headof the first available data associated with the burst of data words. 17.The memory controller integrated circuit of claim 16, wherein saidmemory controller integrated circuit is to transmit the information tothe memory device as part of the read command.
 18. The memory controllerintegrated circuit of claim 10, wherein the memory controller integratedcircuit is to sample the burst of data words output from the memorydevice at the memory controller based on information provided by thespecified timing reference signal output from the memory device.
 19. Amemory controller integrated circuit to control dynamic random accessmemory (DRAM) devices corresponding to respective ranks of memory,comprising: circuitry to transmit a read command to one of the DRAMdevices, the read command to cause the one of the DRAM devices to outputdata accessed from a memory core via a data bus common to the DRAMdevices, the data to be output by the one of the DRAM devices as a burstof data words; circuitry to, provided that the burst of data follows aperiod of specified duration during which the memory device was notexchanging data with the memory controller integrated circuit via thedata bus, transmit information to the memory device which specifieswhether the memory device is to commence outputting of a specifiedtiming reference signal prior to commencement of outputting of the burstof data; and receiver circuitry to receive the timing reference signalif the information specified that the one of the DRAM devices output thetiming reference signal; wherein the timing reference signal comprises atoggling pattern having at least four bits.
 20. The memory controllerintegrated circuit of claim 19, wherein the memory controller integratedcircuit further comprises phase update circuitry to update at least onephase value respective to the one of the DRAM devices in response toreceipt of the toggling pattern by the memory controller integratedcircuit, said memory controller integrated circuit to use the at leastone phase update of a timing signal used to exchange data with the oneof the DRAM devices via the data bus.
 21. The memory controllerintegrated circuit of claim 19, wherein the transmission of theinformation to the memory device is performed in the event that the readcommand corresponds to a rank switch relative to data last exchangedwith the memory controller integrated circuit via the data bus.
 22. Thememory controller integrated circuit of claim 19, wherein circuitry totransmit the information to the one of the DRAM devices is to issue aregister programming command to a selective one of the DRAM devices toprogram a register of the selective one of the DRAM devices, to causethe output the specified timing reference signal prior to commencingoutput of a burst of data.